1. Field of the Invention
The present invention relates to a method and apparatus which are applied to a conventional closed or bass-reflex speaker system to expand its reproduction range, particularly, to the low frequency side.
2. Description of the Prior Art
A conventional closed or bass-reflex speaker system has an inherent lowest reproduction frequency. This frequency is uniquely determined depending on the volume of a cabinet and characteristics (f.sub.O, Q.sub.O, and the like) of a speaker unit used, and cannot be changed, in particular, expanded.
For example, even if a speaker unit itself is replaced with one which can reproduce a sound to an extremely low frequency, a closed speaker system has a limitation determined by the volume of a cabinet. As to a bass-reflex speaker system, a cabinet and a unit are normally matching-designed to optimize a so-called phase inversion effect of a port. For this reason, if only a unit is replaced, the optimal matching state is disturbed, and a good result cannot be expected.
A resonance frequency f.sub.OP of a Helmholtz's resonator constituted by a cabinet and a bass-reflex port may be extremely decreased regardless of the basic concept of a bass-reflex speaker system. In this case, in a drive method using a conventional power amplifier, the Q value of the speaker unit is increased and the Q value of the resonator is decreased due to mutual dependency of the speaker unit and the resonator. Thus, a sufficient bass sound resonance radiation power of the resonator cannot be assured.